1. Field of the Invention
The present invention relates to subsurface well devices and methods and particularly to gas generating power charges of the solid propellant type used to actuate the operative components of such devices.
2. Description of the Prior Art
A variety of subsurface well devices are known in the art which require actuation of operative components once the device is positioned at a given depth in the well bore. Such subterranean well devices include packers, bridge plugs, drill stem test tools, tubing hangers, safety and other valves, test trees, and the like. These subsurface well devices have been operated in the prior art by a wide variety of mechanisms. One of the more common methods is by manipulating the tubing string, e.g., pushing and/or pulling, tubular rotation, and the like. Other actuation methods include the use of hydraulic/hydrostatic pressure, as where an actuating fluid is pumped through the bore of the production tubing or work string to the downhole device to actuate the device.
Both of the previously mentioned actuating methods suffer from certain disadvantages. Manipulation of the tubing string can be difficult to accomplish at extreme depths or in the case of deviated wells. The use of through the tubing fluid pressure to actuate down hole devices requires the presence of ports or openings in the wall of the tubing string. Such openings provided in the wall of the production tubing or work string must be effectively sealed against leakage of any fluids subsequently carried in the tubing, such as the produced well fluids. Since the seals that are employed in and between operating components of well tools, such as pistons and housings, are subject to deterioration and leakage, it is difficult to insure sealing integrity. Also the use of hydrostatic pressure is generally not feasible at shallower well bore depths where the available pressure is too low.
One way to eliminate the need for manipulation of the tubing string during actuation procedures is to provide a downhole energy source, such as a gas generating solid propellant or power charge, which can be ignited to provide kinetic energy by the provision of a suitable triggering signal. By mounting the power charge and triggering device in an annular space created on the exterior of the tubing string, e.g. between the exterior of the tubing string and a surrounding cylindrical member, the need for ports or openings in the wall of the tubing string can be eliminated.
The utilization of a downhole energy source which can be transformed into kinetic energy by the provision of a triggering signal to operate a well tool is shown, e.g., in U.S. Pat. No. 3,233,674. The downhole source of energy is an explosive charge which is discharged and the resulting gas is applied to a piston which functions to set a hanger in a well casing. The triggering signals for energizing the downhole circuitry for effecting the discharge of the explosive charge are produced by a pair of sonic frequency generators which are located at the surface and which are transmitted downhole through well fluids or a tubing string.
In spite of these advances, the provision of an annular shaped power charge in a subsurface well device creates special requirements which are not met by presently available power charges. For instance, in the case of a well packer, an elastomeric packing element is mounted in surrounding relationship to the production tubing or work string and is actuated by the downhole apparatus to sealingly engage the surrounding well bore or casing. The speed of burn or gas generation rate of the gas generating charge should be slow enough to allow the elastomeric components, such as the packing elements, sufficient time to compress and assume a packed-off geometry within the well bore. The use of a relatively slow burning solid propellant is therefore preferred since a sudden explosion, accompanied by a sudden release of energy could damage the parts of the apparatus, or provide insufficient stored sealing stress to seal the packing elements.
A need exists for a annular shaped gas generating charge which is particularly adapted for slow actuation of a variety of downhole tools incorporating elastomeric components.
A need exists for such a gas generating, solid propellant charge which has a characteristic speed or burn rate slow enough to allow elastomeric components, such as packing elements, sufficient time to compress and assume a desired geometry without damage to the components.